The present invention relates to protective relays for use in multi-phase AC power distribution systems and more particularly to phase selector relays for detecting faults related to a particular phase of the multi-phase AC power distribution system.
Phase selector relays are employed in protective systems for high voltage transmission lines for detection of the type of fault within the protected portion of the transmission lines. The phase selector for a particular phase, for example phase A, B or C, of a three-phase transmission line may be connected to the secondary current of the transmission line. Each particular phase selector should only detect faults related to its protected phase. For example, a phase selector relay A should only detect phase A when it is the faulted phase or one of the faulted phases. Furthermore, the phase selector for phase A should not be affected by a phase B-to-ground fault, a phase C-to-ground fault, or a phase B-to-C fault. The single phase discriminating detection operations are desired so that a fault related to a single phase will not cause a three-phase circuit breaker to be activated.
In addition, it is desired that the protection of the transmission line remain secure or stable for unfaulted conditions. For example, during the open pole time or period which the faulted phase or phases are disconnected from the transmission line, it is desired that the phase selector detect and responsd to a fault which may develop on an unfaulted phase of a transmission line. Also, it is desired that the phase-selector device does not operate incorrectly under maximum line-loading conditions, and it is preferable that the phase selector operate only for faults in the selected zones of the transmission line.
In the prior art one means of phase selection has been the use of single phase distance relays. A major disadvantage of distance relays is their inability to detect high resistance faults which dramatically limits the sensitivity of the scheme. A further problem of distance relays is the tendency, under very heavy load conditions, to select more than one phase, or the incorrect phase on single line to ground faults, thus requiring analysis to establish settings that preclude misoperation.
Another means of phase selection on single line to ground faults is the use of directional units comparing the phase relationship of the negative and zero sequence current components. These work well on short lines, but tend to become unreliable for remote end faults on long lines where the magnitude of the zero sequence component of current can be very small. With series capacitors, a remote end fault can result in a phase reversal of the zero sequence current relative to the positive and negative sequence currents, thus requiring analysis to establish settings of supervising overcurrent units to preclude incorrect operation should the series capacitors cause an apparent phase reversal in the zero sequence current. Further, the directional units will be unreliable on faults involving more than one phase. On double line to ground faults they select the unfaulted phase, and on line to line faults, the zero sequence component in the fault will be zero, but may exist in the relay due to CT errors, etc. Thus, the prior art phase selector had to be supplemented with another form of phase selection to provide three pole tripping on interphase faults. Typically this form of phase selection used a compensated positive sequence under voltage relay, which became very difficult to set with series capacitors in the line section.
Accordingly, it is an object of the present invention to provide a phase selector relay that accurately determines the phase or phases which are directly involved in the abnormal fault condition.
It is another object of the present invention to provide a phase selector relay, adapted to be coupled to a high voltage transmission line, which operates in a manner such that a fault related to a single phase of the high voltage transmission line does not cause multiple phase tripping.
It is a further object of the present invention to provide protection of the transmission line that remains secure for unfaulted conditions.
It is yet another object of the present invention to eliminate the need for settings in a phase selector relay.
It is still a further object of the present invention to provide a phase selector relay which operates on current only, for easier application on series compensated lines.
These and other objects of the present invention will become apparent to those skilled in the art upon consideration of the following description of the invention.